Polymeric membranes for separating mixtures of gases, such as methane and carbon dioxide are known. For example, U.S. Pat. Nos. 7,247,191; 6,932,859; 6,755,900; 7,981,974; 8,066,799; 8,337,598; 8,394,182; and 8,328,906, which documents are incorporated by reference herein in their entireties, teach crosslinkable polymers and crosslinked hollow fiber membranes made from such crosslinkable polymers. These patents particularly describe a crosslinkable polyimide polymer. The crosslinkable polyimide polymer can be made by monoesterifying a polyimide polymer with a crosslinking agent.
A crosslinked hollow fiber membrane can be made by forming fibers from the crosslinkable polyimide polymer and transesterifying the crosslinkable polyimide polymer within the fibers. More specifically, the crosslinkable polyimide polymer can be formed into crosslinkable fibers, which are then subjected to transesterification conditions to create covalent ester crosslinks between the crosslinkable polyimide polymer within the fibers. Crosslinked hollow fiber membranes can be incorporated into a separation module. Other types of membranes for separation include flat sheet separation membranes or flat stack permeators.
Separation modules utilizing hollow fiber membranes include a larger surface area than separation modules utilizing flat sheet or flat stack permeators. Therefore, hollow fiber separation modules have significant separation capability even in a reasonably compact size module. Module size is important in implementing separation modules on offshore platforms, where space and weight are at a premium, to separate mixtures of gases from hydrocarbon producing wells.
The crosslinked hollow fiber membranes have good selectivity; however, the transesterification conditions to create covalent ester crosslinks between the crosslinkable polyimide polymer within the fibers causes a huge drop in permeance. The permeance loss can be, for example, about 50% or even as high as around 70% or higher. Crosslinking is also a difficult step to perform on commercial scale.
Therefore, there remains a need for a method of making a high molecular weight, monoesterified polyimide polymer which retains its selectivity and permeability. The monoesterified polymer also needs to have good strength, flexibility, and/or spinnability. Further there is a need for making separation membranes having improved permeance and selectivity.